I think it depends on the song. Some allow for more variation in different performances.

In listening to field recordings of unaccompanied traditional singers, I've noticed that each stanza scans a little differently and the tune varies. I've noticed that even so, there is a consistency in different performances with very little change.

Sometimes I believe discoveries are made which alters the performance of a song. There are those rare times when this happens on the spot. A sudden insight will reveal a new meaning for a word which changes the whole meaning of the song.

I suspect that whenever a song is sung, it is interpreted in some fashion. Even a complete disregard for its content might be an interpretation.

A professional performer will tend to work toward a consistency that has been tried before an audience and found successful. This may not be the case with jazz singers who might change according to which musicians are supporting them. Traditional songs require a minimum of variation since the lyric is generally the most important aspect. Too much variation or vocal "jamming" could obscure the lyric.

Shakespeare can be reinterpreted many different ways but generally is consistent with the interpretation of the director. A trad ballad or song requires a similar consistency to keep it from too much complexity.

This brings into question a larger issue, that of innovation vrs. tradition. How do the two work together?

Frank Hamilton